Thanks for elaborate information , I really liked
it . Still why do u need to provide db name in tnsnames, all one should know
is which instance to connect . Instance know , which db to connect
.
why do net8 bother about dbname ?

db_name - identifies database name, has to be
the same what is stated in controlfile (using create database or create
controlfile). You can have several databases with same db_name in one
server, there is no restriction.

SID - When we actually want to open and use the
database, we have to start an instance which will be servicing the
database. When starting instance, whe have to specify SID (system
identifier) for it. This is actually just a operating system name for
instance (or should I say SGA shared memory segments). As you know, SID is
specified using ORACLE_SID OS environment variable and is only used by
listener, when spawning new processes or when attaching directly to SGA
using bequeath protocol. You can not have instances with same SID in one
server, even if you use different oracle homes.
instance_name - An Oracle parameter for
specifying Oracle instance name. Seems that operating system doesn't know
anything about it. For example, if you use SID_LIST parameters in your
listener.ora, then listener always knows how to spawn processes for given
SID, because (almost) all it takes to start another server process, is the
location of oracle executable and SID value. But if you want to connect
using instance name (not SID itself) then Oracle instance actually has to
register itself with listener before any server processes can be spawned.

Note that ORACLE_SID and instance_name
variables do not have to match. (Tested on 9.2.0.1 on W2K). If my SID is
ORCL for example and instance_name is TEST, then I can connect using both
SID=ORCL and SID=TEST in CONNECT_DATA section of TNSNAMES (again, the
instance name has to be registered with listener). V$INSTANCE still shows
SID (ORCL) in instance name fielt.

As we know, in OPS and RAC environment you
could have several instances servicing one database, this is one
of common reasons why db_name and instance_name would be different.
(ORCL for db_name, ORC0, ORC1, ORC2, etc.. for instance names for example).
Btw, does anyone know if I can have the same instance name in all RAC nodes?
RAC shouldn't care much, because it uses instance numbers
anyway?

service_names - this is an
additional layer for logically spreading work across nodes in RAC (and
possibly other load balanced environments such replicated databases and even
logical standby). For example, if you have 4 node RAC and have defined two
"services" such OLTP and REPORTING, then you could assign OLTP for nodes and
REPORTING for one, but during month end reporting you could still keep OLTP
on 3 nodes, 1 node purely for REPORTING and add REPORTING to one or more
OLTPs as well. So, when user's are connecting, they don't request a specific
node or instance, they just request a service. And in load balancing
environments then users are directed to instances, where appropriate service
is defined. It gives somewhat flexibility, but I've never used it myself
(thus my explanation here might wrong ;)
Cheers,
Tanel.
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To: <A [EMAIL PROTECTED]
href="mailto:[EMAIL PROTECTED]">Multiple recipients of list
ORACLE-L
Sent: Wednesday, July 02, 2003 9:49
PM
Subject: service name, sid .
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I am reading oracle network admin guide and
getting confused abt service name, instance name , db name , sid .
why service name is not same as db
name . Earlier service name and sid used to be same thing .. isn't it
( ? ) .
Can some one clarify with some examples .
TIA
-ak
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